1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic copying machine or a laser beam printer, which includes a developing device that develops an electrostatic latent image formed on an image bearing member into a toner image.
2. Description of the Related Art
A magnetic brush developing method that uses a developing sleeve which is a developer bearing member as a two-component magnetic brush developing device is generally used. A developer that includes powder of a magnetic material (for example, a magnetic carrier which is a ferrite) and toner formed by dispersing pigments in a resin, for efficiently developing an electrostatic latent image on a photosensitive drum is agitated and mixed so that charges are held on toner by friction charging due to mutual friction.
This developer is held on a developing sleeve which is a hollow cylindrical developer bearing member made of a nonmagnetic material having magnetic poles in the inside thereof. The developing sleeve carries the developer from a developer container to a development area facing the photosensitive drum, and in this development area, the developer is caused to form a magnetic brush by the action of the magnetic field to rub the surface of the photosensitive drum. As a result, the electrostatic latent image formed on the photosensitive drum is developed.
The two-component magnetic brush developing method using this developing sleeve is used in many products such as a monochrome digital copying machine and a full-color copying machine that requires high image quality.
In an image forming apparatus using such a developing device as described above, contamination of the inside of the apparatus due to scattering toner is a problem. When a developer flies between the photosensitive drum and the developing sleeve, toner floats up in the air to become scattering toner. In this case, the scattering toner leaks outside the developing device from upper and lower gaps present between the developing device and the photosensitive drum.
Since an LED, an optical system, and the like, or a transfer unit, a conveying path, and the like are often disposed above or below a development device, various members may malfunction or deteriorate and a toner stain or the like may remain on an output image.
Conventionally, a technique of applying a scattering toner prevention bias is known as a technique of preventing toner from scattering toward the upstream side in the rotation direction of a developing sleeve (Japanese Patent Laid-Open No. 2010-231017 and U.S. Patent Application Publication No. 2010/0247164A1). In this technique, a scattering preventing electrode is disposed so as to prevent toner from scattering from the inside of a developer container. Further, the scattering preventing electrode is disposed vertically above the developing sleeve and closer to the downstream side in the rotation direction of the developing sleeve than a straight line passing the two points which are the center of rotation and the apex of the developing sleeve.
Moreover, Japanese Patent Laid-Open No. 2000-112237 proposes a technique of a scattering toner collecting roller as a technique of preventing toner from scattering toward the lower side of a development device. According to the technique of Japanese Patent Laid-Open No. 2000-112237, the collecting roller is disposed on the downstream side in the rotation direction of a developing sleeve at a position where the developing sleeve makes contact with a photosensitive member.
The collecting roller to which a bias voltage is applied rotates in a reverse direction to the developing sleeve. Toner scattering from a development area is deposited or attracted onto the collecting roller located under the development area. The toner deposited to the collecting roller is conveyed with rotation of the collecting roller and is scraped by a scraper and collected into a developer container. In this manner, the toner scattering from the developing sleeve is prevented from leaking outside the developer container.
Moreover, conventionally, discharge control is performed so that deteriorated toner in the development device is discharged during a non-image forming period. Specifically, toner is discharged to a photosensitive member in a state where a developing bias in which an AC bias and a DC bias are superimposed is applied to a developing sleeve. Moreover, Japanese Patent Laid-Open No. 2008-139400 discloses that discharge control is performed using a DC developing bias.
However, in a developing device including such a scattering preventing bias electrode as described above, an electrode for applying a scattering preventing bias is disposed in a developer container and a high-voltage substrate (or high-voltage rectifier board) for applying a scattering preventing bias is disposed. Thus, there is a problem that the cost and the size of the development device increase.
Moreover, when the conventional deteriorated toner discharge control is performed by a discharge operation based on an AC+DC developing bias, it may be difficult to efficiently discharge toner lumps which become the cause of scattering toner as will be described later.
Thus, when discharging is performed using a DC developing bias only as in Japanese Patent Laid-Open No. 2008-139400, since the discharging is not executed in order to prevent scattering toner, the execution frequency (execution condition) is uniform. Moreover, toner rarely scatters in an initial stage of use. Thus, if the DC discharge control is performed uniformly as in Japanese Patent Laid-Open No. 2008-139400, toner may be consumed unnecessarily.
Moreover, the following problems occur when the discharging operation is performed using a DC developing bias. That is, developing performance decrease as compared to when the discharging operation is performed using an AC developing bias. Thus, the downtime may extend when deteriorated toner other than toner lumps which become the cause of scattering toner is discharged.
Thus, it is desirable to satisfactorily prevent toner scattering while suppressing the downtime and a toner consumption amount with an inexpensive configuration.